DIY Back Up Power For a Sump Pump

by Stephan on August 2, 2010

One of the most terrible things to happen during a power outage is your basement flooding because the sump pump doesn’t have any power. You’re already in basement because of a Tornado Watch and the sirens are going. Of course, the power usually goes out during bad storms when plenty of water is falling out of the sky, which just makes it worse, helping your sump fill up faster. You don’t really want to do a flooded basement repair as they are expensive and preventable, if you take a little time and effort now by making a homemade battery backup.

We are going to assume that you are home for the power outage to plug things in. If you want a automatic sump pump backup system, there are ones on the market for this precise use. We are going on the idea you are sheltering in your basement because a tornado or something is roaming through town and going outside to start a generator is a very bad idea, but the sump is still filling with water.

The first thing to do is figure out how much power your sump pump uses. All electric sump pumps have a nameplate that shows the name of the manufacturer, the model number, the serial number, but more importantly the Voltage and Current it uses, some even show the sump power watts rating. If it does not show its power consumption in watts you’ll have to calculate it, which is very easy. Just multiply the Voltage in Volts usually 110 or 120 by the Current in Amps to get Watts.

Typically, a 1/3 HP sump pump will use 7 Amps of Current, that would be 120V x 7A=840 Watts when running. Something important to remember is that since it is a motor it will have a high starting current, this is also known as a surge or peak current and typically it is twice the running wattage or 2 x 840W = 1680 W peak. Of course, a heavy duty sump pump will use more power.

Once you get to about 1 hp sump pump it’ll be too much for a battery backup to handle. You might want to consider a water powered backup sump pump, which uses the pressure in the city water main to run the pump.

This gives us an idea of how powerful an inverter we need to get. You can get a 1000W inverter at a home improvement store for about $80. That is 1000W running and 2000W surge. You don’t want to run an inverter at full power for very long, it will last much longer if you only use it at 80% of its rating.

The second thing is to how long the cellar pump runs for. You probably have a feel for how often it comes on, they tend to be a little noisy and can dim the lights when they come on, that is their startup surge.

To get a really good idea of how much power it uses, you can get a Kill-A-Watt meter from your local home improvement store or electronics store. Plug the meter into the wall and the sump pump into the meter. Note the date and time and come back after 72 hours. If you’re like me it is best to set a reminder on the computer or cellphone. I am going with 72 hours because most blackouts are much shorter then that but if it is because of a disaster help should be getting there by that time, with generators and fuel and so on.

Then all you have to do is press the KWH button and it will show you how many kiloWatt-hours the pump used in that time. You take that number and divide by 120 Volts to find out how many Amps-hours it used during that time.

For example, if the Kill-A-Watt meter read 21 kWhs. How big a battery would you need? 21 kWh รท 120 V = 176 Amp-hours. Again we don’t want to over-discharge the battery so we should gives ourselves a 20% safety factor. 176 Ah x 1.2 = 211 Ah. The nearest and larger size of battery is a 220 Ah battery.

A word on batteries: Batteries, particularly those with high amp hour ratings, can be dangerous. Like fire it needs to be treated with care and respect. The laws of physics don’t care and will kill you if you get in their way. The most important thing to remember is your heart does not like electricity going through it. Work with one hand in your pocket at all times and know where the electricity is wanting to go. Electricity is like water, it seeks the easiest path to lowest potential energy, in this case, earth ground. Standing on an insulator like rubber shoes, rubber or wooden mat will also help to protect you.

Generally speaking there are three kinds of batteries: Starter, deep cycle and marine.

Starter batteries are the ones you find in most cars. They are designed for very high current output for very short amounts of time, typical a few seconds. They do not like to be discharge more then 2-5%. This makes then unsuitable for this application. You can use them but if you discharge them more then 20% they will be damaged and will stop working after only a few cycles.

Deep Cycle batteries are the kind you use on boats, golf carts and RVs to power things when the engine isn’t running or their is no engine. They are designed to be discharged 80% over a long time usually measured in hours or days. They can be damaged if you try to start a car off of one.

Marine batteries are a hybrid between the starter and deep cycle batteries. They can be used to start a motor and also be used all day to keep the radio, lights and cooler running.

We want a 12 V Deep Cycle or Marine Battery for this application, and we determined its capacity above. Get one with a handle, it will make moving the battery much easier.

Safety Warning: Batteries contain lead and sulfuric acid, both are very nasty. To minimize your risk get a Sealed battery.

Third, how to charge the battery. You are going to need a battery charger. One from the automotive store will do just fine if it is a smart charger, which means that it will slow down or trickle charge the battery so it doesn’t overcharge. Because we are using a sealed battery you will want to charge it using its lowest setting. It will take longer but will reduce the risk of the battery overheating, giving off hydrogen and having the emergency pressure release tab popping and spraying sulfuric acid everywhere.

Read, understand and follow the battery charger manufactures instructions but generally all you have to do plug the charger into the wall, hook the alligator clamps, they look like small jumper cables, to the battery positive to positive and negative to negative and set it to charge on its lowest setting, especially if you are using a sealed battery, remember. Generally, it will take overnight.

Batteries tend to self-discharge over time. A battery holds the charge in a chemical solution and it will slowly react with itself. As a rule of thumb you’ll need to recharge the battery once a quarter. If you have a wet season, definitely recharge before it starts.

Battery backup sump pump installation

Since this is dealing with electricity and we remember that electricity and water don’t mix, we need to put the battery, charger and inverter on a shelf, not on the floor. If you have kids a lockable cabinet would be a very good idea. Or you can keep the battery and charger in the garage until you need and carry it down then, that handle will come in handy then. Just keep the inverter neat the sump pump so you don’t lose it.

If your crawl space sump pump installation has an easy to reach hatch, this will work too, if not then a pre-made emergency backup sump pump is a better way to go, you don’t want to be crawling around under the house during a major storm with a blackout.

All you really need to do is put the battery next to the inverter. Hook the inverter to the battery using the alligator clamps, positive to positive and negative to negative. Unplug the sump pump from the wall and into the inverter. And away it goes.

That should hold off the water so you won’t need a basement flood repair.

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